The fabrication of semiconductor devices including field effect transistors on silicon wafers that are planar or non-planar structures such as silicon nanowires requires methods of doping or activating devices that are not hindered by the lack of planarity. Plasma doping means dopant precursors are added to a gas stream such that the number of dopant atoms is in rough proportion to the number required on the wafer to obtain the desired dose. The feed gas stream includes an inert carrier gas and is very lean with the precursor gas. The precursor gas is dissociated into free dopant atoms in a background plasma and the free atoms along with inert carrier gas ions impinge the wafer, energetic ions amorphize (disorder the first few atomic layers) the surface allowing the dopant atoms to be taken up by the surface. Annealing following the doping process activates the dopants in the sub-surface.
Plasma doping has as its objective the incorporation of dopant species into the silicon sub-surface with control of electrical resistance and junction depth over the non-planar structure. Plasma doping ensures an adequate dose distributed over an appropriate depth in the silicon sub-surface with minimum damage, minimum sacrifice of the active device integrity, and compatibility with subsequent process steps. A key challenge for plasma doping is obtaining adequate dose with good uniformity in the presence of clustering phenomena that occur under high dose conditions.
Some general ways to achieving low sheet resistance include: a) increasing of flow rate of process gas including dopant; and b) increasing of RF bias power on wafer stage. Both simple solutions bring with them significant problems. High dopant flow rates result in dopant clustering, which actually results in dopant loss; flow fields of the dopant coming in may also produce significant uniformity problems. Radio frequency (RF) bias at the wafer leads to erosion by energetic ions and decreasing controllability of the depth of the dopant, referred to as the junction depth, Xj.
The clustering of dopant atoms occurs in arsenic, phosphorus and boron doping processes when the dopant concentration exceeds a critical value. In the silicon sub-surface, if dopant atoms concentration exceeds this critical threshold value, the dopant atoms bond to neighboring dopant atoms and vacancy, and form “clusters”. In the annealing process, the clusters, usually volatile at elevated temperatures, sublime with loss of dose. Any remaining clusters are electrically inactive and have a non-diffusive character. In arsenic doping, a typical cluster consists of four arsenic atoms and a vacancy. There are many different kinds of clusters. Whatever the kind, they reduce the number of effective carriers and inhibit dopant diffusion. Finally, sheet resistance is increased due to a decreasing of effective carrier density and shallow diffusion depth. To achieve high dose amount and low sheet resistance, processes that inhibit dopant clustering and promote dose uptake are needed.
“Oxidation enhanced diffusion” means that oxygen atoms incorporated in silicon enhance dopant diffusion and inhibit dopant clustering. This phenomenon is known to be effective during process and during annealing. We have re-confirmed that a small amount of oxygen atoms co-incorporated in silicon with dopant atoms also promotes dopant incorporation in plasma doping. With oxidation of the silicon sub-surface during the annealing process, oxygen atom interferes silicon atoms bonding, switches positions with non-bonding silicon atom, and bonds other silicon atoms. Many interstitial silicon atoms are generated. The interstitial silicon atoms unite with vacancies, and decrease vacancy density. The result is that arsenic atoms (exemplary of other dopant species) are unable to find partners (: vacancy) to form the clusters. Clustering is inhibited.
Stress mediated diffusion also plays a role. Oxidation of a surface layer results in a stress field propagating into the film. This effect is seen in other fields such as plasma etches where the presence of an oxide results in “bird's beak” effect at oxide-silicon interfaces. Stress mediated diffusion promotes dopant diffusion.
There are several problems with current additive addition methods. Typically, additive addition methods are used in the annealing step where oxygen gas is added to the annealing ambient which comes with significant problems. Too much oxidation on the silicon surface or other chamber surfaces may occur requiring a cleaning process that lowers throughput.
Introduction of additives in the gas stream is a tempting approach to control additives. The problem of adding oxygen or other precursors into gas streams is that without special solutions, for example, fast gas switching, it is difficult to control the uniformity of the flux of the additive to the wafer. A solution is required to add oxygen or other dopant additives to the plasma doping process as simple solutions such as addition of oxygen to the gas stream, RF bias, and high dopant flow rates are inadequate on their own to provide high dose uniformly across a wafer. There is also a fundamental drawback associated with adding oxygen in the gas stream. Adding oxygen (molecules) in any appreciable amount also increases the degree of electronegativity of the plasma through attachment processes.